A chemical reaction that can lead to damage in concrete structures. Free lime (CaO) in cement reacts with CO2 in the atmosphere to precipitate CaCO3 around the cement grains. This protects them from weathering and also gives an alkalinity level (pH higher than 7.0) which helps to protect steel from corrosion. If the aggregate contains soluble silica, however, new minerals may precipitate by reaction between the aggregate and the cement. These may absorb water, causing the concrete to swell and eventually crack. Water entering these cracks may cause rusting of reinforcement bars and repeated wetting and drying may eventually destroy a structure.